First attempt at ripping out agnostic code into ifcopenshell.api namespace. See #1399.

src/ifcopenshell-python/ifcopenshell/api/root/reassign_class.py

@@ -0,0 +1,26 @@
+import ifcopenshell
+import ifcopenshell.util.schema
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "product": None,
+            "ifc_class": "IfcBuildingElementProxy",
+            "predefined_type": None,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        element = ifcopenshell.util.schema.reassign_class(
+            self.file, self.settings["product"], self.settings["ifc_class"]
+        )
+        if self.settings["predefined_type"] and hasattr(element, "PredefinedType"):
+            try:
+                element.PredefinedType = self.settings["predefined_type"]
+            except:
+                element.PredefinedType = "USERDEFINED"
+                element.ObjectType = self.settings["predefined_type"]
+        return element